This invention relates to electrostatic chucks having improved erosion resistance, for holding substrates in process chambers.
In semiconductor fabrication processes, electrostatic chucks are used to hold a semiconductor substrate, such as a silicon wafer, during processing of the substrate. Electrostatic chucks are generally described in, for example, U.S. patent application Ser. No. 08/189,562, entitled "Electrostatic Chuck" by Shamouilian, et al., filed on Jan. 31, 1994, which is incorporated herein by reference.
Electrostatic chucks can be formed from as little as a single electrostatic member comprising an insulative layer with an electrode embedded therein. However, a typical electrostatic chuck includes a base capable of being secured to a support in a process chamber with the electrostatic member on the base. More typically, the electrostatic member on the chuck has coolant grooves therein for holding a coolant for cooling the substrate to prevent overheating and damage to the substrate during processing. To use the chuck, a substrate is placed on the electrostatic member, and the electrostatic member is electrically biased with respect to the substrate by an electrical voltage. Process gas is introduced into the process chamber for processing the substrate, and in certain processes, a plasma is formed from the process gas. Opposing electrostatic charge accumulates in the electrostatic member and in the substrate, and the resultant attractive electrostatic force electrostatically holds the substrate to the chuck. The electrostatically held substrate covers and seals the coolant grooves on the chuck so that coolant does not leak out from the grooves.
The electrostatic member on the chuck typically comprises an insulator having one or more metallic electrodes embedded therein. An insulated electrical connector strap extends over the edge of the base and connects the electrode of the electrostatic member to a voltage supply source. Typically, the insulator on the electrode and on the connector strap comprises an electrically insulative organic polymer, such as a polyimide. The use of polymers to insulate the electrode and connector strap of the chuck, limits the lifetime of the chuck in certain substrate fabrication processes. Polymers have relatively low erosion resistance for certain process gases and plasmas, and in particular, for oxygen-containing gases and plasmas which are used for a variety of substrate processing operations, including etching of substrates and cleaning of process chambers. When a substrate is held on the chuck, a portion of the polymeric insulator on the chuck is covered by the substrate and protected from the erosive gases in the chamber. However, the insulator at the perimeter of the electrostatic member and on the electrical connector strap portion that extends over the edge of the base of the chuck is exposed to the erosive gas in the process chamber. After numerous processing cycles, the exposed insulator erodes until the electrode or the electrical connector is exposed to the plasma. Exposure of the electrode or connector may occur in as few as a thousand process and cleaning cycles. Exposure at even a single point may cause arcing between the electrode and plasma in the chamber, destroying the usefulness of the chuck and requiring replacement of the entire chuck. Frequent replacement of chucks is expensive and slows down the fabrication process. Also, if the chuck fails during processing of the substrate, the entire substrate can be lost, at a cost of several thousands of dollars.
Although alternative insulative materials, such as silicon oxide based ceramic insulators, can be used to insulate and protect electrostatic members in oxygen-containing processes, these materials have limited effectiveness in other processes. For example, silicon oxide insulators rapidly erode in processes that use fluorine containing gases, such as fluorocarbon gases. Also, ceramic insulators are generally more difficult and expensive to manufacture.
Conventional electrostatic chucks have another disadvantage. A typical electrostatic chuck can have an electrostatic member comprising (i) a single electrode for use as a monopolar electrode, or (ii) two or more electrodes for use as bipolar electrodes. Monopolar electrode chucks can electrostatically hold a substrate only during the plasma stages of the process. Typically, a positive potential is applied to the monopolar electrode on the chuck, and the substrate is maintained at a negative potential by the charged plasma species in the chamber which impinge on the substrate, causing electrostatic charge to accumulate in the substrate and electrically hold the substrate to the chuck. During the non-plasma stages of the process, the substrate is not electrostatically held to the chuck and can move or become misaligned during processing. Also, because the substrate is not clamped to the chuck, the substrate does not seal the coolant grooves on the chuck, preventing holding of coolant in the grooves for cooling the substrate.
Bipolar chucks can electrostatically hold a substrate for both plasma and non-plasma processes because bipolar chucks are operated by applying a positive potential to one of the electrodes and a negative potential to the other electrode. However, when a bipolar chuck is used in a plasma process where charged plasma species impinge on the substrate, a complex circuit is necessary to balance the current flow, and voltage applied to, each electrode. The complex circuit requires a floating power supply that allows the charged plasma species to maintain the substrate either at a negative potential of a few hundred volts or at electrical ground. The complex circuitry renders the chuck more complicated and costly to use.
Use of conventional bipolar electrode chucks is also disadvantageous because bipolar electrodes provide only about 1/4 of the electrostatic clamping force provided by a monopolar electrode having the same electrode area. Lower electrostatic force occurs because electrostatic clamping force is proportional to the square of the electrode area, and each bipolar electrode has only half the area of a single monopolar electrode. Maximizing electrostatic clamping force is advantageous because higher clamping force reduces movement or misalignment of the substrate during processing which can result in loss of the entire substrate. Also, the higher clamping force allows more coolant, or coolant at higher pressures, to be held in the cooling grooves without coolant escaping from the grooves, thereby allowing better control of the temperature of the substrate.
Thus, it is desirable to have an electrostatic chuck that is substantially resistant to erosion in corrosive gaseous environments. It is also desirable to have a chuck that can maximize the electrostatic clamping force for holding the substrate to the chuck. It is further desirable to have a chuck that can be used for both non-plasma and plasma processes without use of complex circuitry and that can be inexpensively fabricated using conventional fabrication equipment.